1. Field of the Invention
The present invention relates to a method for making a polycrystalline silicon thin film, and more particularly to a method for making a polycrystalline silicon thin film by crystallizing an amorphous silicon thin film at a low temperature.
2. Description of the Prior Art
Recently, polycrystalline silicon (p-Si) thin films have been used for p-Si TFT liquid crystal displays (LCDs) and contact image sensors (CISs), in dynamic random access memory (DRAM) and display fields. A method which are currently used in making such p-Si thin films comprises heat treating an amorphous silicon (a-Si) thin film or a microcrystalline silicon (.mu.-Si) thin film at a high temperature of about 1,000.degree. C. A p-Si thin film produced according to this method exhibits a superior characteristic, but encounters a phenomenon that its substrate is wraped and shrunk due to the heat treatment at the high temperature. As a result, the method undesirably requires the use of expensive substrates made of a single crystal silicon (c-Si) or a quartz. In particular, the phenomenon becomes more severe at a larger thickness of the thin film. In severe cases, alignment of pattern as a subsequent process cannot be carried out.
To solve these problems, there have been proposed techniques for crystallizing an a-Si thin film to form an p-Si thin film at a low temperature of not more than 600.degree. C. According to these techniques, a glass substrate can be used for making the p-Si thin film, so that the manufacture becomes inexpensive.
As such low temperature crystallization techniques, there are three methods, that is, a solid phase crystallization (SPC) method, a rapid thermal annealing (RTA or RTP) method and a laser annealing (LA) method.
The SPC method has an advantage that an a-Si thin film of a large area can be easily uniformly crystallized to form a p-Si thin film, However, it requires a heat treatment at about 600.degree. C. for a long period of 24 hours to 72 hours, Due to such a long crystallization period, the glass substrate which has a deformation temperature of about 580.degree. C. is likely to be wraped and shrunk, Such a wraping phenomenon may be greatly exhibited, depending on materials deposited over the substrate and their deposition thicknesses. in cases of making TFT devices using a semiconductor integration process, this phenomenon also causes a problem in a pattern alignment. Consequently, it serves as a factor causing a degradation or loss in characteristics of the element finally obtained. The long crystallization period results in a decrease in productivity and an increase in a possibility that impurities contained in the glass substrate will penetrate the p-Si thin film formed over the glass substrate.
The RTA method is a process enabling a crystallization within a short period of several seconds to several minutes, as compared with the SPC method. However, this method encounters a problem in maintaining a constant temperature and a substrate deformation caused by a high temperature exposure.
On the other hand, the LA method has an advantage that a p-Si thin film exhibiting a high drift mobility can be made within a short period. However, this method has a problem in obtaining a uniform crystallization, since the crystallized area obtained at one crystallization is small.
As apparent from the above description, all conventional low temperature crystallization methods have respective severe problems. Accordingly, a new method capable of solving the problems encountered in the conventional methods is required.